This invention relates to solid state electronic relays and more particularly to such relays utilizing photovoltaic isolators and field effect power transistors.
Solid state relays are well known for use in electrical power systems to control the energization of a load by a power source. In direct current systems, the switching element of a solid state relay usually takes the form of a transistor switching circuit as shown in U.S. Pat. No. 3,898,552, issued Aug. 5, 1975.
In order to achieve increased isolation between control and power circuits, photovoltaic isolators have been used. A solid state relay utilizing an optical isolation technique is disclosed in U.S. Pat. No. 3,321,631, issued May 23, 1967. Since optical isolators could not supply sufficient power to drive the output transistor of a solid state power relay directly, relays using optical couplers exhibited a high switch drop. Transformer-oscillator drive circuits were developed to provide sufficient driving power, as disclosed in U.S. Pat. No. 3,710,231, issued Jan. 9, 1973. This resulted in a design choice between relays with a low switch voltage drop which utilized bulky transformers and relays with a high switch voltage drop which used optical couplers.
The availability of power field effect transistor (FET's) and optical isolators which develop sufficient voltage to turn on these FET's has provided means for improving solid state relay performance. U.S. Pat. No. 4,227,098, issued Oct. 7, 1980, describes a solid state relay which incorporates a power field effect transistor and photodiode optical coupler.
In various applications, different modes of operation are required from a solid state relay. U.S. Pat. No. 4,188,547, issued Feb. 12, 1980, disclosed a multi-mode control logic circuit for solid state relays with provisions for normally open, normally closed, and latched operation in a single circuit. A particular operating mode was selected by connecting a separate mode terminal to line voltage or ground or by leaving the mode terminal unconnected.